1. Field of the Invention
This invention relates generally to an improved catalyst for air purification systems. More specifically, the present invention relates to a catalyst of an air purification system for decreasing the deactivation rate of active oxides by silicon-based airborne contaminants.
2. Description of the Related Art
Some buildings utilize air purification systems to remove airborne substances such as toluene, formaldehyde, propanal, butene and other contaminants from the air supply. These substances are generally known as volatile organic compounds, or VOCs. By removing VOCs from the air supply, building operators can save on energy costs by circulating cleansed air and reducing the quantity of fresh air brought into the building while maintaining acceptable air quality and potentially providing an improved environment.
Photocatalysis is a proven method for the removal of gaseous airborne contaminants such as VOCs. A photocatalytic air purification system uses a photocatalytic reactor that is comprised of a substrate which is coated with a photocatalyst that interacts with airborne oxygen and water molecules to form hydroxyl radicals when irradiated by appropriate light source. This light source is typically an ultraviolet (UV) light source. The hydroxyl radicals formed attack the VOCs and initiate the oxidation reaction that converts them into less noxious compounds, such as water and carbon dioxide. It is further believed that the combination of water vapor, suitably energetic photons, and a photocatalyst also generates an active agent such as a hydrogen peroxide that can act over a distance of several microns from its point of generation. This active agent also contributes to the oxidation of the organic contaminants.
A commonly used photocatalyst is titanium dioxide, otherwise referred to as titania. Degussa P25 titania and tungsten dioxide grafted titania catalysts, such as tungsten oxide on P25, have been found to be especially effective at removing VOCs when irradiated by a UV light.
One problem with photocatalytic air purifiers is the deactivation of the photocatalyst caused by the mineralization of volatile compounds containing silicon, especially the class of compounds known as siloxanes. Where the aggregate amount of VOCs in air is typically on the order of 1 part per million by volume, siloxane concentrations are typically two or more orders of magnitude lower. Volatile siloxanes in buildings arise primarily from the use of certain personal care products or dry cleaning fluids; they can also come from the use of silicon caulks, adhesives and the like. The hydroxyl radicals released by the photocatalysts attack and oxidize the siloxanes, to form solid non-volatile silicon dioxide or hydrated silicon dioxide that deactivates the photocatalyst. This deactivation occurs by physical blockage of the active photocatalyst sites by the oxidation products. This occurs when the non-volatile silicon compounds directly block the pores in the photocatalytic coating that would otherwise allow access by the other airborne VOCs to the active photocatalyst sites. The deactivation can also be indirect, by blocking the interaction of the VOCs with the catalyst active sites.
The blocking of the photocatalyst sites by the silicon compound contaminants can significantly reduce the lifetime of the photocatalyst, which fail when the photocatalyst sites are blocked. This can represent a significant cost if the photocatalyst requires frequent replacement
Therefore, there is currently a need for an air purification system that is siloxane resistant and on which siloxanes have a significantly reduced effect.
It is an object of the present invention to provide a photocatalyst that can be immune to or resistant to the effects of airborne silicon contaminants in a commercial air purifier.
It is a further object of the present invention to provide such a photocatalyst that can reduce the costs associated with the maintenance of currently available purification systems.